New in the box or NIB should mean exactly that new and unfired , it is still second hand , which can cause warranty problems, and if in current manufacture should be sold for approximately wholesale price , unless it has a rarity value .
So the problem with this, is many firearms are test fired before being sold by the manufacturer, and some come with a test target.
HOW does one know the "wholesale" price? Most if not all of the manufacturers won't tell you that price, unless you yourself are an actual business AND in some cases, there are two wholesale prices. The first is for a single unit or several, and the other is for bulk ordering.
I am thinking lack of availability ( Covid pricing ) is more at play here than anything else?
Actually it's demand vs. availability which may be amplified by COVID, plus quality, and that's then impacted by condition.
IF factory made muzzleloaders, and cap-n-ball revolvers were just sitting in warehouses, then they'd go on sale to move them. You mentioned Classifieds here..., you can see on some of the items the price drops over time when few or no inquiries are made about the item.
Add to that quality. A well known manufacturer or a well known custom maker, and an item that is very well made, boosts that price. You payed more for a Hatfield or a Remington flintlock, even though both were made by Pedersoli and are Frontier rifles... because Hatfield and Remington had Pedersoli do more to the rifles' finish and the choice of wood, than merely changing the name on the box.
Provenance can up a price too. I saw an old, beat up, vintage pistol for 10X what the price should be, at a gunshow. Asked the seller, and he showed me the documents that went with the pistol showing it had once belonged to a rather well known historical person. Celebrity use would also a factor..., although not authentic, one of the rifles used by Fess Parker when he played Daniel Boone or Davey Crockett, would likely fetch a pretty price. How about the 1858 Remington used by Clint Eastwood in Pale Rider?
When it's a private individual, you get a lot of quirks too (imho). I've seen folks asking current pricing for factory made muzzleloaders, and in their photos you can see the serial number, and check the manufacture date. They are asking top retail dollar for a thirty year old, used gun. Even if it was just a wall hanger in fantastic shape, it's not under warranty. SOME want more than the current retail price, having convinced themselves that theirs is somehow "collectable", just due to age. What's bothersome is that the current "used going rate" for what they are selling is normally equal to if not more than what they paid thirty years ago (shipping and sales tax included), but they somehow take offence when that price is offered.
Some quirky thinking is worse. For example I saw a used Perdersoli Bess for sale, at slightly less than current retail price. Obviously a restocked musket, because the 42" barrel was there, but the Shmendrik ran the stock up to the muzzle and omitted the nose cap. When I asked if they had the nose cap, and offered less because I was going to need to redo the stock so the barrel would take a bayonet, I got a very nasty reply about how the musket was "done by a well known builder" (he failed to mention who) and was "authentic"... "denial" is also a river in Egypt....,
Lastly is condition, which we all know. A beat-up Seneca won't go for nearly as much as one that looks like it came right out of the box, today. Sometimes this works in the buyer's favor though, when it's reversed. I have a "defarbed" Pedersoli Bess. It has the after market, rounded brass sideplate resembling a LLP Bess, and was retro fitted with a butt plate with much longer brass on the stock comb. The engraved date on the lock was changed to an earlier date, so that the musket would more resemble a modified LLP Bess (lots of LLP's were shortened from 46" barrels down to 42"). It had a lot of surface rust, and the seller was "dumping" it..., and I got a very low priced used Bess, and after an application of Evaporust, and some polishing of the brass, she's good to go.
LD